SUMMARY African Americans develop kidney failure at rates 4-5 fold higher than European Americans. Coding variants in the APOL1 gene, found only in people of recent African ancestry, drive a large fraction of this risk disparity. We have named these APOL1 variants G1 (S342G and I384M) and G2 (del388N389Y). Risk inheritance is recessive: only individuals carrying two variant APOL1 alleles have a markedly increased risk of kidney disease. In the U.S. approximately 13% of African Americans, or about 4,000,000 people, have a high-risk genotype. Most recessive diseases are caused by loss-of- function mutation. Surprisingly, there is data from model systems in kidney cells, mice, drosophila, and yeast that support the idea that the APOL1 risk variants (RV) are actually gain-of-function mutations that cause toxicity when expressed at high levels. The overarching goal of this proposal is to understand the mechanism of APOL1 kidney disease such that two RVs are required, providing insight into this fundamental question of recessive, gain-of-function toxicity. We propose that the G0, or wild- type, APOL1 allele can alter the behavior of the risk variants and thus prevent their toxicity. In AIM 1, we will use isogenic, Crispr-engineered APOL1 BAC transgenic mice to determine the differing properties of G0 and risk-variant APOL1 in the full complexity of the glomerulus. We will test the effects of different APOL1 variants singly and in combination in order to answer questions about gene dosage, WT rescue of risk-variant toxicity, and possible differences in mechanism of action between the two major APOL1 risk alleles (G1 and G2). In AIM 2, our goal is to understand APOL1 targeting to lipid droplets. We have observed that lipid droplets (LDs) show prominent G0 but little or no G1 or G2 localization in cell systems and that co-expression of G0 facilitates the movement of G1 or G2 onto lipid droplets. We will use imaging and biochemical analyses to examine determinates of APOL1 localization to LDs, determine if there are differences in the binding of APOL1 to other LD-associated proteins, and define the cellular consequence(s) of altered APOL1 risk variant targeting to the LD. We will test the relevance of these experiments with in vivo studies. In AIM 3, we will determine the role of aggregation in APOL1-mediated cytotoxicity. We have observed that the APOL1 RVs have much stronger propensity than G0 to aggregate. Our data indicate that RV aggregation occurs in mitochondria. We hypothesize that aggregation is a key element in RV toxicity, and that G0 can interfere with RV aggregation. In cell-based systems we will characterize the APOL1 aggregates, determine whether aggregation is essential for APOL1-mediated cytotoxicity, understand the effect of aggregates on mitochondrial function, and test whether G0 can reverse RV aggregation and cytotoxicity. In vitro results will be validated in kidney organoids, a mouse model, and in human tissues.